1. Field of the Invention
The present invention generally relates to ferromagnetic thin film memories and sensors and, more particularly, relates to wordline keepers for thin film magnetoresistive memory devices and sensors.
2. Description of the Prior Art
Digital memories of various kinds are used extensively in computer and computer system components, digital processing systems and the like. Such memories can be formed, to considerable advantage, based on the storage of digital bits as alternative states of magnetization of magnetic materials in each memory cell, typically thin-film materials. These films may be thin ferromagnetic films having information stored therein based on the direction of the magnetization occurring in those films. The information is typically obtained either by inductive sensing to determine the magnetization state, or by magnetoresistive sensing of each state.
Such ferromagnetic thin-film memories may be conveniently provided on the surface of a monolithic integrated circuit to thereby provide easy electrical interconnection between the memory cells and the memory operating circuitry on the monolithic integrated circuit. When so provided, it is desirable to reduce the size and increase the pack density of the ferromagnetic thin-film memory cells to achieve a significant density of stored digital bits.
Typically, a thin-film magnetic memory includes a number of bit lines intersected by a number of word lines. At each intersection, a thin film of magnetically coercive material is interposed between the corresponding word line and bit line. The magnetic material at each intersection forms a magnetic memory cells in which a bit of information is stored.
A number of competing factors influence the packing density that can be achieved for such a memory. A first factor is the size of the memory cells. The size of the memory cells must typically decrease with increased packing density. Reducing the size of the memory cell, however, can reduce the magnitude of the magnetic field that can be produced by the word and sense line currents in each memory cell.
A second factor is the width and thickness of the word lines. The dimensions of the word lines must typically decrease with increased packing density. Reducing the dimensions of the word lines, however, reduces the current that can be accommodated thereby, and thus the magnetic field at the corresponding magnetic bit region.
A third factor is the distance between the word lines, and thus the distance between a wordline and an adjacent memory cell. Typically, the distance between word lines must decreases with increased packing density. However, this increases the possibility that the magnetic field produced by one word line may adversely effect the information stored in an adjacent memory cell.
U.S. Pat. No. 5,039,655 to Pisharody discloses one approach for reducing the magnetic field cross-talk between a word line and an adjacent memory cell. Pisharody discloses providing a magnetic field keeper formed from a superconductor material around at least three sides of the word lines. Pisharody states that the superconductor material may shunt the magnetic fields generated by a word line, and thus reduce the effects thereof on an adjacent memory cell.
In Pisharody, a number of bit lines are deposited on an insulating layer, wherein each bit line includes a thin metallic conductor surrounded by a film of magnetically coercive material. An insulating layer is then deposited over the bit lines. A number of parallel word lines oriented generally perpendicular to the bit lines are then provided over the bit lines. The magnetic material at the intersection of the bit lines and the word lines define the memory cells.
A thin insulating layer is deposited over the word lines which separates a thin film of superconducting material from the word lines. Because the thin insulating layer follows the contour of the word lines, the superconducting material covers three sides of each word line. Pisharody states that the superconducting material shunts the magnetic fields generated by the word lines, thereby allowing the word lines to be packed closer to one another in the array structure.
One limitation of Pisharody is that the word lines and corresponding vias are formed after the magnetic film. For minimum size and pitch, metal deposition, and in particular via processing, must typically be performed at temperatures that may damage or otherwise degrade the magnetic properties of the magnetic material. This is particularly true for metal processes the use tungsten or other similar material. Thus, in Pisharody, the word line metal and vias may have to be formed at non-minimum dimensions, or alternatively, formed using non-conventional fabrication techniques which are performed at reduced temperatures. Thus, it would be desirable to provide a word line structure where the word line metal and all vias are provided before the magnetic material of the memory cell, thus allowing the use of conventional fabrication techniques, when desired.
Another limitation of Pisharody is that the dielectric layer between the magnetic film on the bit lines and the word lines must be relatively thick. This is primarily because the dielectric layer is deposited over a relatively non-planer surface, including the bit lines. This reduces the effective magnetic field produced by the word line at the magnetic film. Thus, it would be desirable to provide a word line structure that is relatively planer, thereby allowing a thinner dielectric layer between the word line and the magnetic film.